Remote microphone assemblies such as those used with radio communication equipment (e.g., two-way radios, etc.) are susceptible to radio frequency interference (RFI) caused by the radio when the radio is transmitting information. This interference is due to the close proximity of the microphone assembly to the radio's antenna when it is radiating RF energy. Presently, in order to shield microphone assemblies from RFI or electromagnetic interference (EMI) costly shielding of the microphone assembly is required. This added shielding usually takes the form of specially designed microphone housings or the addition of extra components, such as "desense capacitors", and/or RF chokes, to the microphone assembly.
Such RFI and EMI is found in portable applications and in mobile radio applications where the mobile radio is mounted in the trunk of the vehicle and the microphone assembly is wired through the car into the passenger compartment. In such vehicular installations, the PTT, keypad and microphone signals are very susceptible to interference.
Another problem encountered in some vehicular installations is the voltage drop caused by the extended cable lengths between the microphone assembly and the communication devices. Usually, the keypad and PTT signals generated at the microphone assembly are "read" by an analog-to-digital (A/D) converter circuit in the mobile which is looking for a particular voltage level corresponding to the button presses. When the extended cable lengths are added, the voltage windows detected by the A/D converter are altered due to the voltage drop caused by the cable's resistance. Also, such long cables act as antennas which increase the possibility of picking up unwanted noise.
A need thus exists for a microphone assembly which can provide for immunity from RFI/EMI interference and cable losses, while avoiding the need of expensive shielding techniques.